Water vapor clustering in the field of a chlorine anion occurring in a planar nanopore with structureless walls

Abstract

The Monte Carlo bicanonical statistical ensemble method has been employed to calculate the free energy, entropy, and work of Clion hydration in model planar pores 0.5 and 0.7 nm wide at 298 and 400 K. A detailed model of many body interactions with the ion has been used, the model being matched to exper imental data with respect to the free energy and enthalpy of attachment reaction in water vapor. Under the conditions of a restricted volume, the equilibrium size of a hydration shell substantially decreases, with the effect becoming stronger in the range of moderate and large sizes. In moderately supersaturated vapors, under the conditions of a nanopore, the ion loses its hydration shell as the temperature is decreased. In supersatu rated vapors, the hydration shell formed on the ion is thermodynamically stable, while the stability crisis shifts to the region of larger sizes. The enhancement of the thermodynamic stability in the pore results from a rise in the chemical potential of molecules due to the deficiency of closet neighbors and a reduction in the entropy under the conditions of the restricted volume. As the temperature is elevated, the effect of ion displacement out of its hydration shell is leveled. The regularities derived in terms of the estimation model based on the cap illary approximation are in qualitative agreement with the results of computer simulation.

Structure and electric properties of the hydration shell of a singly charged chloride ion in a nanopore with hydrophilic walls

“…Whereas the relative effect of each factor on the molecular cluster shape can be distinguished with one or another degree of certainty, their joint effect on the orientation of molecules is collective and difficult to separate. Due to the limited space in the nanopore, the molecular cluster is stretched along the pore plane, which is accompanied by the tendency to force out the ion from its hydration shell, as observed earlier in the absence of pore [70,71] or for pores with hydrophobic walls [49,51,52].…”

“…However, later this effect was also reproduced for cations [75,76] which do not exhibit so high polarizability. In the alternative explanation [51], the effect was considered as a result of superposition of several factors, one of the chief factors being the effect of perturbations in the network of hydrogen bonds between molecules. Forcing out the ions to the cluster surface is accompanied by minimization of these perturbations and thus decreases the energy.…”

“…In the present work, computer simulations are used for studying the effect of hydrophilic walls of a flat nanopore on the structure of hydration shells of singly charged chloride ions. In [49][50][51], the same method was used for finding the data on the structure and thermodynamic properties of this system under the conditions of a nanopore with hydrophobic walls. In [52][53][54], the hydration of ion was simulated for the case of hydrophobic walls.…”